1. Field of the Invention
The present invention relates to a liquid crystal display element which can be operated in a display mode selected from a reflective display mode, a combined reflective/transmissive display mode, and a transmissive display mode, and an electric device which is equipped with the liquid crystal display element.
2. Description of the Related Art
In recent years, liquid crystal display devices which display screens using liquid crystal are utilized in every possible display device ranging from small display devices for portable telephones to large display devices for monitors, television sets and the like. A liquid crystal display device comprises liquid crystals sandwiched between two substrates, each of which is formed with electrodes, and displays an image by changing the alignment of the liquid crystal by a voltage applied between the substrates to adjust emitted light.
There exist three types of liquid crystal display devices. One is a transmissive liquid crystal display device which employs a back light for a light source, and displays an image through the liquid crystal layer. Another one is a reflective liquid crystal display device which employs external light such as illumination light, sun light or the like for a light source, and reflects the external light on a liquid crystal layer to display an image. The remaining one is a semi-transmissive liquid crystal display device which has a combination of better image quality inherent in the transmissive liquid crystal display device and higher visibility inherent in the reflective liquid crystal display device.
In particular, portable information terminals such as portable telephones, PDA (Personal Digital Assistance) and the like, which have become rapidly pervasive in recent years, are utilized not only indoors but also outdoors. In addition, they are utilized by day and night. Accordingly, high visibility is required for portable information terminals under any illumination environment. For this reason, the semi-transmissive liquid crystal display device has become prevalent in recent years because it is characterized by having high visibility.
Semi-transmissive liquid crystal display devices are generally divided into two types. One is an internal semi-transmissive type which comprises a reflecting plate within a liquid crystal layer to reflect incident light within the liquid crystal layer, as disclosed in JP1999-242226A from line 25 of column 23 on page 13 to line 19 of column 26 on page 14 and FIG. 1. The other one is an external semi-transmissive type which comprises a reflecting plate external to a liquid crystal layer to reflect incident light after it has passed through the liquid crystal layer. Currently, the internal semi-transmissive type is prevalent because the external semi-transmissive type suffers from parallax (double image) and resulting low visibility due to a substrate disposed between the reflecting plate and liquid crystal layer.
Referring to FIG. 1, an internal semi-transmissive liquid crystal element is illustrated in the cross-sectional view, where a pair of substrates 102a, 102b are disposed opposite to each other above back light source 109, and polarizing plates 101a, 101b are disposed on surfaces of substrates 102a, 102b opposite to their opposing surfaces, respectively. Also, the internal semi-transmissive liquid crystal element comprises reflective display unit 121 which comprises internal reflecting plate 120 in the form of a reflective electrode having ragged reflective surfaces, and transmissive display unit 122 which comprises electrode 103b, on substrate 102b which is disposed closer to back light source 109. Further on an opposing surface of substrate 102a, which opposes substrate 102b, electrode 103a is provided to extend across reflective display unit 120 and transmissive display unit 121, and liquid crystal layer 104 is encapsulated between two substrates 102a, 102b. 
Specifically, an internal reflective semi-transmissive liquid crystal display device configured like the internal semi-transmissive liquid crystal element illustrated in FIG. 1 comprises reflective display unit 121 which comprises internal reflecting plate 120 for reflecting incident light from the outside into one pixel, and transmissive display unit 122 for transmitting light irradiated from back light source 109. In this way, reflected light and transmitted light can both be utilized for displaying an image.
It should be noted that reflective display unit 121 differs from transmissive display unit 122 as regards the appropriate thickness of a liquid crystal. As such, liquid crystal layer 104 is often formed in different thicknesses in the respective areas. In FIG. 1, insulating film 127 is disposed on substrate 102b in reflective display unit 121, and internal reflecting plate 120 is formed on insulating film 127 such that the spacing between internal reflecting plate 120 and opposing electrode 103a in reflective display unit 121 is smaller than the spacing between opposing electrodes 103a, 103b in transmissive display unit 122.
However, in the configuration as described above, since the reflecting plate is formed only in a part of one pixel, a dark display is produced using the reflection of external light, as compared with a reflective liquid crystal display device in which a reflecting plate extends across one pixel.
Thus, JP2002-23156A discloses a method of reversibly switching back and forth between a state which presents high light reflectivity and low transmittance and a state which presents high light transmittance and low light reflectivity, from line 14 of column 5 on page 4 to line 18 of column 7 on page 5 and in FIGS. 1 to 7. JP2004-69835A also discloses such a method from line 5 on page 5 to line 17 on page 10 and in FIGS. 1 to 2.
Referring to FIG. 2, a liquid crystal display device is illustrated in the cross-sectional view, where reflection control layer 108, polarizing plate 101b, glass 107, liquid crystal layer 104, color filter layer 106, color filter substrate 105, and polarizing plate 101a are laminated above back light source 109 in this listed order. Reflection control layer 108 is formed of three-layer laminate HPDLC (Holographic Polymer Dispersed Liquid Crystal) which takes advantage of Bragg reflection the reflectivity of which can be electrically switched. Color filter layer 106 comprises filters for transmitting only R (red), G (green), B (blue), respectively.
This liquid crystal display device can be switched between a reflective display and a transmissive display by reversibly switching reflection control layer 108 between the state which presents high light reflectivity and low transmittance, and the state which presents high light transmittance and low reflectivity. In this way, the liquid display device can provide a full color reflective display in a light environment, and a full color transmissive display in a dark environment.
Referring to FIG. 3, a liquid crystal display device is illustrated in the cross-sectional view, wherein polarized-light selection reflective layer 111, liquid crystal layer 112, polarizing plate 101b, glass 107, liquid crystal layer 110, color filter substrate 105, and polarizing plate 11a are laminated above back light source 109 in this listed order. Polarized-light selection reflective layer 111 reflects light of a particular linear polarization, and transmits light of linear polarization orthogonal to the above-mentioned linear polarization.
In a reflective display as illustrated in the left-hand cross-sectional view of FIG. 3, when a voltage is applied to liquid crystal layer 112 to align liquid crystal molecules of liquid crystal layer 112 perpendicularly to the layer surface, light incident on liquid crystal layer 112 does not change in polarization. Accordingly, the incident light is reflected by polarized-light selection reflective layer 111 for use in a reflective display.
On the other hand, as illustrated in the right-hand cross-sectional view of FIG. 3, when no voltage is applied to liquid crystal layer 112, liquid crystal molecules of liquid crystal layer 112 are aligned in parallel with the layer surface, and are also twisted in alignment. In this state, light incident on liquid crystal layer 112 rotates by 90°.
Accordingly, when external light passes through liquid crystal layer 112, the external light is transmitted through polarized-light selection reflective layer 111 and cannot be utilized for display. However, within light which reaches polarized-light selection reflective layer 111 from back light source 109, light which vibrates perpendicularly to the surface of the drawing sheet reflects on polarized-light selection reflective layer 111, while light which vibrates in parallel with the surface of the drawing sheet is transmitted through polarized-light selection reflective layer 111 and impinges on liquid crystal layer 112. The light which impinges on liquid crystal layer 112 has its polarization direction rotated by 90° by liquid crystal layer 112, and is not absorbed by but passes through polarizing plate 101b. Thus, when no voltage is applied to liquid crystal layer 112, the external light is utilized for transmissive display.
Also, JP1998-206844A discloses a display method which involves switching a semi-transmissive selective light reflection layer over an entire screen to one of a reflection mode and a transmission mode.
The techniques described in JP2002-23156A and JP2004-69835A are the techniques to switch the display device between the reflective display mode and transmissive display mode to improve reflectivity in a reflective display. However, the reflection control layer, or the reflecting plate made up of the second liquid crystal layer and polarized-light selection reflective layer, is disposed outside of the liquid crystal layer for displaying an image. This gives rise to a problem of parallax (double image) experienced in a display, causing a degradation in visibility in a reflective display.
On the other hand, in the method described in JP-1998-206844A, the selective light reflection layer, which is disposed over the screen, is semi-transmissive per se. This gives rise to difficulty in creating a suitable optical design for the reflective display unit and transmissive display unit.
Further, the foregoing techniques have a problem of the inability to select an optimal display mode under a variety of illumination conditions which differ from one use environment to another.